1. Field of the Invention
This invention relates to phase shifters and more particularly to microwave phase shifters.
2. Description of the Prior Art
Conventional phase shift circuitry for phase shifts greater than 221/2.degree. uses reflection-type circuitry since large phase shifts are difficult to achieve with loaded line circuits. Conventional phase shift circuitry may use a perfect or assumed perfect 3 decibel hybrid coupler, with intervening circuitry between the coupler and the diodes to provide the desired phase shift. By using a 3 decibel hybrid coupler and circuitry between the coupler and the diodes, constraints are placed on the diodes and the intervening circuitry to such an extent that the desired characteristics of the phase shift circuitry such as phase flatness, voltage standing wave ratio (VSWR), insertion loss, and amplitude modulation cannot be realized in a repeatable manner among a number of manufactured phase shifter circuits.
In U.S. Pat. No. 4,205,282 which issued on May 27, 1980 to J. W. Gipprich and assigned to the assignee herein, a phase shifting circuit element was described using reflection-type circuitry. FIG. 1 shows a 3 dB coupler having four ports with 2 ports coupled through respective reactive networks 22 and 24 to respective diodes 18 and 20. In FIG. 6 a conventional branch line quadrature coupler 76 is shown having parallel line pairs 82 and 84 coupled between port 2 at 90 and PIN diode 94. Line pairs 86 and 88 are coupled between port 3 at 92 of hybrid coupler 76 and PIN diode 98. Parallel line pairs 82-84 and 86-88 are adjusted in length such as a quarter wavelength or less to provide a predetermined phase shift.
In U.S. Pat. No. 4,105,959 which issued on Aug. 8, 1978 to V. Stachejko, a hybrid coupled phase shifter is described for introducing a predetermined phase shift in a microwave frequency signal. A pair of transmission lines 18 and 20 are interconnected at their centerpoints by a branch transmission line 22 having approximately the same width as lines 18 and 20, and interconnected at their respective extremities by narrow branch transmission lines 24 and 26. Branch transmission lines 22, 24 and 26 are approximately one-quarter wavelength long at the center frequency of operation, and transmission lines 18 and 20 are approximately one-half wavelength long. Diodes 32 and 34 are coupled at the end of transmission lines 18 and 20 respectively to provide an open or a shorted termination to transmission lines 18 and 20. Highly resistive material 56 through 59 is disposed on transmission lines 18 and 20 as shown in FIG. 1 to provide energy absorbers for balancing the insertion loss through the circuit between times when the diode is conducting and non-conducting.
In U.S. Pat. No. 3,789,329 which issued on Jan. 29, 1974 to G. E. Johnson, an 8-bit digital phase shifter is shown in FIG. 1. Phase shifting networks 40, 42 and 44 use a 3 dB hybrid and PIN diodes which are forward bias at times to provide a predetermined phase shift.
It is therefore desirable to provide a phase shifter element utilizing a 4-port coupler which has transmission lines of various impedances and lengths.
It is further desirable to provide a phase shifter element utilizing a 4-port circuit which incorporates transmission lines of various impedances and lengths at the mid-band frequency which results in fewer constraints on the diodes.
It is further desirable to provide a phase shifter circuit element using a 4-port circuit which selected transmission line impedances and phase lengths for a predetermined phase shift which has phase flatness over a band of desired frequencies, a low voltage standing wave ratio, a uniform insertion loss across the band, and low amplitude modulation.
It is further desirable to provide a phase shifter using a hybrid circuit having transmission line lengths and impedances selected to provide 3.5 decibel maximum insertion loss, .+-.0.5 decibel maximum amplitude modulation, and a 1.5:1 maximum input and output VSWR.